Single-photon absorptiometry has become widely accepted for precise and accurate determination in vivo of bone mineral in both biomedical research and clinical practice. A new development, dual-photon absorptiometry, now permits measurement of total body bone mineral with high precision and accuracy (2 percent error). The method involves scanning over the entire body with a source of 153-Gadolinium and a scintillation detector; the dose is about 0.1 mrad. These scan measurements permit the quantitation of the mass of mineral and the mass of soft tissue at each point in the body; at locations where there is no bone mineral the relative lean-fat composition of the soft tissue is determined. The scan data can be outputted via computer to a graphics display which provides quantitative images of the distribution of bone mineral, soft tissue, lean tissue, fat tissue, and/or other combinations of these. The mass of bone mineral, lean and fat may be determined for an anatomical area of the body (trunk, limbs, etc.). We propose to continue refinement of this method, and to apply the method to the study of normal subjects and patients. The Wisconsin Bone Mineral Laboratory has pioneered in development of single and dual-photon absorptiometry. We will work on optimization of the dual-photon absorptiometry technique with particular emphasis on developing computer algorithms for imaging. Studies will be done of long-term precision. Cross validation of absorptiometric determinations will be done by measuring subjects who have had determinations of total body calcium (by neutron activation) or body composition performed. Measurements will be made on normal subjects, and on a limited number of patients, to define normal age-sex patterns and the deviations due to disease. The absorptiometric measurements will be made together with emission bone scans (using 99mtc) in order to provide a bone image and to correct the emission scans for attenuation and scatter.